When people communicate with one another, some cultures use lots of background knowledge and special information that isn't said out loud. This is called a high-context culture.
Imagine you have a secret handshake with your best friend. You've known each other for a long time, and the handshake means something special to you both. If you went to a different country where people didn't do secret handshakes, you might have trouble communicating with people because they wouldn't understand what your handshake meant. In a high-context culture, there are lots of secret handshakes, and people rely on their shared knowledge and history to understand each other.
On the other hand, some cultures are low-context. This means that people rely on language and clear communication to get their point across. In a low-context culture, you wouldn't need a secret handshake to communicate with someone because you could just use words to explain what you're thinking.
Think of it like building a puzzle. In a high-context culture, everyone starts with some pieces already put together. They understand each other without having to explain everything in detail. In a low-context culture, everyone starts with only the puzzle pieces, and they have to put them together by talking to each other and asking lots of questions.